A polymer electrolyte membrane fuel cell largely consists of an end plate, a current collector plate, a bi-polar plate, a gas diffusion layer, a metal nanoparticle catalyst layer, and a polymer electrolyte membrane, and among them, a gas diffusion layer of positive and negative electrodes, a platinum-supported carbon black catalyst layer of positive and negative electrodes, and a polymer electrolyte membrane are collectively called a membrane electrode assembly (MEA).
This MEA takes up about 70% of a material cost of a polymer electrolyte membrane fuel cell stack and is the biggest obstacle to the commercialization of a polymer electrolyte membrane fuel cell. In addition, the platinum catalyst among elements constituting the MEA takes up the highest portion of the price. Accordingly, the US Department of Energy (DOE) sets a goal that the amount of platinum (Pt) used will be reduced from about 0.3 mg/cm2 in the current level to 0.05 mg/cm2 or less by 2025.
In order to reduce the amount of platinum used, it is necessary to thin the thickness of a platinum catalyst layer and concentrate the platinum catalyst to regions adjacent to a membrane layer such that a catalytic reaction which ionizes hydrogen may usually occur between the catalyst layer and the membrane layer. However, a platinum layer may not be selectively formed on a specific site in the catalyst layer by a spray method or a decal method, which is a conventional method of forming a platinum nanoparticles catalyst layer.
Specifically, the platinum nanoparticle catalyst layer of a conventional polymer electrolyte membrane fuel cell was manufactured by dispersing a carbon black power in a platinum compound aqueous solution, adding a reducing agent to the dispersion to reduction-deposit platinum nanoparticles on the carbon black 10, separating the platinum-supported carbon black from the solution to wash the platinum-supported carbon black, mixing the platinum-supported carbon black with a mixed solution of the polymer electrolyte solution and a solvent to prepare a platinum-supported carbon black slurry 130, applying the slurry on a carbon paper 140 as a gas diffusion layer by a spray method or a decal method to manufacture platinum nanoparticle-supported carbon black sheet electrodes 140 and 150, installing the platinum-supported carbon black sheet manufactured on and below a polymer electrolyte membrane (M) of a membrane electrode assembly (MEA), and using a high temperature press to perform a hot pressing (100° C.).
In order to be immobilized on a carbon paper which is a gas diffusion layer in the manufacturing process of a platinum catalyst layer, the platinum-supported carbon black powder is suspended in a Nafion solution which is a polymer electrolyte to be applied on the carbon paper by a spray method or a decal method. In this case, the platinum metal catalyst is buried in the polymer electrolyte as in FIG. 2 to fail to exhibit the performance of the catalyst sufficiently. Due to these problems, a platinum catalyst layer was applied thicker than necessary, and as a result, this has been responsible for the inability to reduce the amount of a platinum catalyst used.